St. Matthias was the Apostle chosen to replace Judas Iscariot in Acts 1:21-26.

Therefore it is necessary that of the men who have accompanied us all the time that the Lord Jesus went in and out among us— beginning with the baptism of John until the day that He was taken up from us—one of these must become a witness with us of His resurrection.” So, they put forward two men, Joseph called Barsabbas (who was also called Justus), and Matthias. And they prayed and said, “You, Lord, who know the hearts of all men, show which one of these two You have chosen to occupy this ministry and apostleship from which Judas turned aside to go to his own place.” And they drew lots for them, and the lot fell to Matthias; and he was added to the eleven apostles. – Acts 1:21-26.

Little is known of his missionary labors, but tradition states that he preached in Judaea, Cappadocia, and on the shores of the Caspian Sea. He was stoned to death in Jerusalem or was martyred at Colchis or at Sebastopolis. St. Helen brought his relics to Rome, and some were transferred to Trier, Germany.